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In 1950 Physicists Enrico Fermi and Michael Hart were chatting in the Los Alamos canteen when the topic turned to UFOs. Where were they? After a few calculations Fermi felt the probability of alien life was high enough; we just didn’t have any evidence ‘they’ were out there.
Frank Drake built on this in 1961. The Drake equation examined the probabilities of how many stars and planets could host life that could become intelligent and travel in space.

Life on Earth meant the probability of intelligent aliens must be more than zero, but how much more?
Answers to the Drake equation vary, depending on the probabilities you assume. But arguments about the Fermi paradox have sparked a number of ideas about extraterrestrial life (or the lack of it) as well as a number of science fiction plots.
One theory involves the Great Filter: there are stages in the evolution of life that are very hard to pass through. For example abiogenesis (the transition of non-living matter into simple cellular life) may happen very rarely. We still can't replicate it in a test tube!
There may be many Great Filters to intelligent life, so the odds of getting there are slim - evolution doesn’t care if we’re smart! Michael Crichton’s The Andromeda Strain (1969) suggests our first encounter with aliens would be with microbes that had no DNA, RNA or amino acids.
The Great Filter could mean we’re the first, or even only, intelligent life so far in the universe. In Ursula K LeGuin’s Hainish novels life on other planets all comes from one ‘human’ world, Hain. Earth is a lost colony of Hain, with which it eventually makes contact.
More worryingly the Great Filter may be ahead of us: there’s something that prevents most intelligent life ever reaching the stars. For example in The War Of The Worlds (1898) it’s the alien immune system that stops the superior Martians from conquering Earth.
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